This invention relates to integrated circuits, and more particularly to an FET (field effect transistor) assembly adapted to compensate for gate leakage current inherent in the FET and thereby cancel input bias current.
One of the advantages of FET's in applications such as operational amplifiers is that very little input bias current is drawn at room temperature (20.degree. - 25.degree. C). Within this temperature range the input current is typically in the range of 20 to 30 picoamperes. However, the input current has been found to approximately double for every 10.degree. C increase in temperature due principally to increased leakage from the FET gate epitaxial region to its isolation boundary, and can reach 20 to 30 nanoamperes at an ambient temperature of 125.degree. C. In addition, power dissipation in the chip can cause the actual chip temperature to exceed the ambient temperature by up to 30.degree. C, pushing the input current to well over 100 nanoamperes at chip temperatures of 150.degree. C. This is approximately two orders of magnitude worse than the input current characteristic of the better bipolar input operational amplifiers currently available.